Plasma-beam signal generator



Feb. 23,1965 R. TARG ETAL PLASMA-BEAM SIGNAL GENERATOR Filed Dec. 15, 1959 INVENTORS RUSSELL TAEG BffORR/S ETTE/VBEPG ATT RNEY i I. 552/25 A \\\\\7// fl//l /l Il l lf/VII /l l//l//l/ United States Patent O 3,171,053 PLASMA-BEAM SIGNAL GENERATOR Russell Targ andV Morris Ettenberg, New York, N.Y., assignors to Sperry Rand Corporation, Great Neck,

N.Y., a corporation of Delaware Filed Dec. 15, 1959, Ser. No. 859,764 8 Claims. (Cl. S15-3.5)

The present invention relates to apparatus for generating electrical signals and, more particularly, to a signal generator of the plasma-beam type selectively operative over a broad band of signal frequencies, especially microwave frequencies.

Electronic apparatus for the generation and amplification of electrical signals are well known in the art. Through the development of microwave techniques, signal generators are now available for operation over relatively broad bands of frequencies in the microwave region. Such apparatus, however, often includes external circuit arrangements for providing signal feedback to sustain the generated electrical oscillations. Additionally, tuning provisions are relatively complex.

It is the principal object of the present invention to provide a simplified signal generator selectively operative over a broad range of frequencies including microwave frequencies.

Another object is to provide a microwave signal oscillator requiring no external feedback circuits. l

A further object is to provide a readily tunable signal generator of the plasma-beam type.

These and other objects of the present invention, as

will appear from a reading of the following specification,

An electron gun is provided at the other end of said v cylinder for directing an electron beam axially along the cylinder and through the interaction structure toward an insulated beam collector. The cylinder is adapted to contain a controlled pressure of a suitable ionizable. gas atmosphere, for example, hydrogen. The pressure preferably is of the order of 10-4 millimeters of mercury. Solenoid means are mounted exterior to the cylinder for producing a magnetic field axially along said cylinder.

A plasma is formed in the cylinder between the regions occupied by the electron gun and the interaction structure as a result of the bombardment of the gas by the beam electrons. The charged particles comprising the plasma oscillate in the presence of the axial magnetic field. The plasma-contained oscillations velocity modulate the electron beam as it penetrates the plasma. The modulated electron beam then enters into the interaction structure and produces a traveling wave of electromagnetic energy in the helix thereof `in a conventional manner. The frequency of thetraveling wave is a function of the magnitude of the axial magnetic iield; the ampli. tude of the traveling wave is related to the pressure of the gas atmosphere.

For a more complete understanding of the present invention, reference should be had to the following specification and to the drawing which is a simplified sectional diagramV i of a typical embodiment.

Referring to the drawing, the numeral 1 designates a hollow metallici cylinder, for example, a'hollow metallic cylinder of stainlessrsteel. An electron gun 2 is provided at one end of thecylinderfor producing an electron beam directed axially along the cylinder.` .The electron beam is collected by electrically insulated collector 3. A conventional traveling wave tube structure 4 is provided intermediate gun 2 and collector 3. Traveling wave tube i 3,171,053 Patented Feb. 23, 196,5

ments 5 and 6, a helical conductor '7, input line 8 and output line 9. Solenoid 10 is mounted about cylinder 1 and produces a magnetic field parallel to the axis of cylin` der 1.

The interior of cylinder 1 is lilled to a predetermined pressure with a suitable ionizable gas 11, such as, for example, hydrogen. As will be seen more fully later, proper operation of the present plasma-beam signal generator depends upon the maintenance of the predetermined gas pressure level within cylinder 1. To achieve such a result, gas generator 12, valve 13 and pressure gauge 14 are provided. Valve 13 is interposed between input gas manifold 15 and duct 16 which leads to a vacuum pump (not shown). Gas generator 12 may include titanium hydride and a heating lament for dissociating the hydride to release hydrogen. By the manipulation of valve 13, the energization of the heating filament of gas generator 12 and the monitoring of vacuum gauge 14, a desired pressure of the hydrogen within cylinder 1 may be established. In the preferred embodiment, said pressure is of the order of 104 millimeters of mercury.

The electron beam generated by gun 2 ionizes the molecules of hydrogen gas 11 by collision. Secondary electrons are produced in the course of collision. The secondary electrons and the collision-deflected beam electrons will take on substantial velocity components in a direction transverse -to the axis of cylinder 1 The transversely moving electrons-Will experience the well known Lorentz force under the influence of the axial magnetic field and Will undergo cyclotron oscillation at a frequency dependent on the charge-to-mass ratio of the electrons and the strength of the magnetic field.

It is believed that the electron beam on passing through the oscillating plasma is velocity modulated coherently with the plasma oscillations. The lvelocity modulated beam continues its axial movement along cylinder 1 and enters into the input section of traveling Wave interaction structure 4. As a result of the well'known growing wave phenomenon, a traveling electromagnetic wave will appear in helix 7. The traveling wave is coupled out by means of coaxial output line 9. It should be'noted that interaction structure 4 plays no part in the initiation of the oscillatory signal within theplasma. Structure 4 serves merely to amplify and couple out the oscillatory signal which is coupled from the plasma and applied to structure 4 by the axially moving velocity modulated electron beam. Accordingly, alternative means such as, for example, a resonantV cavity, may be provided for extracting the oscillatory signal in the velocity modulated electron beam. The traveling wave tube structure is used in the preferred embodiment because of its well known broad band signal propagating properties.

It has been observed experimentally that a microwave signal in the frequency range from about .5 to 2 kilomegacycles will appear at output line 9 when the pressure of the hydrogen gas 11 is at least of the order of 104 millimeters of mercury.V Gas pressure in excess of said value only increasesthe amplitude of the mcirowave signal. The frequency of the output microwave signal, however, may be varied conveniently by adjustment of the magnitude of the axial magnetic field produced by solenoid 10.

The plasma-beam signal generator of the present invention is readily adapted for operation as a selective signal amplifier.V That is,-if an input'microwave signal is applied via'input line` 3, theapparatus will selectively amplify said input signal to the substantial exclusion of other frequency input signals 'depending on the ina'gni# tude of the axial magneticiield. It has been found ythat maximum power gain of an input microwave signal'is achieved when the frequency of said input signal approaches the frequency, determined by the magnitude of the axial magnetic field, at which the device would oscillate of its own accord. Thus, if the magnetic field is adjusted so that the device is on the threshold of selfsustained oscillation at a predetermined frequency, an output signal will -appear at output line 9 when the frequency of the input signal applied via line 8 substantially equals said predetermined frequency. lFor the same setting of the magnitude of the axial magnetic field, input signals of frequencies other than `the predetermined frequency will suffer substantial discrimination. v

It can be seen, based on the theory of operation proposed above, that the frequency of the output signal generated by the apparatus of the present invention is indicative of the charge-to-mass ratio of the cyclotronically oscillating particles of the plasma. That is, the experimentally observed microwave frequency Vsignals on output line 9 are of a frequency predicted by theoretical consideration of electrons moving in a direction transverse to the applied magnetic field. It may be anticipated that where the moving charged particles of interest have a charge-to-mass ratio other than that of an electron, a different frequency output signal will appear on output line 9. Hence, the apparatus of the present invention is adapted for application as a mass spectrometer.

In the mass spectrometer application of the present invention, gas generator 12 would produce an unknown gas Whose identity is to be determined. Inasmuch as the frequency of the output signal appearing on line 9 is proportional to the charge-to-mass ratio of the charged particles oscillating within the plasma, so the identity of oscillating gasions can be ascertained from the observed frequency of the output signal. It is believed that the same theory applies to the case of the mass spectrometer as was previously described for the microwave frequency generator. The only essential difference, i.e., the lower frequency of the lspectrometer output signal, is attributable to the relatively greater mass of the gas ions with respect to that of electrons.

Thus, it can be seen that the apparatus of the present invention is readily adaptable not only for the generation and amplification of microwave frequency signals but for the identification of `the natureV of the gas atmosphere contained within cylinder y1. In both cases, the frequency of the output signal is dependent upon the magnitude of the axial magnetic field and the charge-to-mass ratio of the charged particles moving within the plasma. A knowledgeof any two of the three parameters, particle charge-to-mass ratio, magnetic field magnitude and output signal frequency, will yield the value of the third parameter.

While the invention has been described in its preferred embodiments, it is understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is: 'Y

1. Apparatus rcomprising a hollow cylindrical member, an electron'beam collector located at one end of said member, an electron gun located at the other end of said member for directing an electron beam axially along said member toward said beam collector, a signal interaction structure positioned intermediate said electron gun and said beam collector in said member for interacting with said electron beam when modulated to produce an output signal having a frequency related to the frequency of the modulated Velectron beam, a gas source connected to saidmember for .producing a controlled pressure of an ionizable gas atmosphere within said member, and means positioned adjacent said member for producing a magnetic field directed axially along said member.

`2. Apparatus comprisinga hollow metallic cylinder, an electron beam collector positioned at one end of said i cylinder, an electron gun positioned atthe other end of said cylinder for directing an electron beam axially along said cylinder toward said beam collector, a signal interaction structure positioned intermediate said electron gun and said beam collector for interacting with said electron beam when modulated to produce an output signal having a frequency determined by the frequency of the modulated electron beam, a gas generator connected to said other end of said cylinder for producing a pressure of the order of 10*4 millimeters of mercury of an ionizable gas within said cylinder, and means mounted about said cylinder for producing a magnetic .field axially along said cylinder.

3. A signal generator comprising a hollow metallic cylinder, an electron beam collector positioned at one end of said cylinder, an electron gun positioned at the other end of said cylinder for producing an electron beam directed axially along said cylinder toward said beam collector, a signal interaction structure of the traveling wave type positioned adjacent said one end of said cylinder and intermediate said electron gun and said beam collector, a gas source and a gas pump connected to said other end of said cylinder for maintaining a controlled pressure .of the order of 104 millimeters of mercury of an ionizable gas within said cylinder, and means for eS- tablishing a magnetic field directed axially along said cylinder.

4. Apparatus as defined in claim 3 wherein said hollow metallic cylinder is a hollow cylinder of stainless steel.

5. Apparatus as defined in claim 3 wherein said gas is v hydrogen.

6. Apparatus comprising a hollow metallic cylinder, an electron beam collector positioned at one end of said cylinder, an electron gun positioned at the other end of lsaid cylinder for directing an electron beam parallel to the axis of said cylinder toward said beam collector, means connected to said cylinder for establishing a pressure of the order of 10-4 millimeters of mercury of an ionizable gas within said cylinder in the region between said electron gun and said beam collector whereby a plasma is formed in said region upon the bombardment of said gas by said electron beam, means for producing a magnetic field directed axially along said cylinder, the charged particles comprising said plasma oscillating at a frequency determined by the magnitude of said magnetic field and modulating said electron beam as said electron beam penetrates said plasma, and means positioned adjacent lthe modulated electron beam within said cylinder for interacting therewith to produce an output signal of a frequency related to that of said modulated electron beam.

7. Apparatus as defined in claim 6 wherein said means for interacting with said modulated electron beam is a microwave signal interaction structure of the traveling wave type having an input and an output helix terminal, said input terminal being adapted to receive an input microwave signal, said input microwave signal being amplified and appearing at said output terminal with an amy Vlitnde determined by the magnitude of said magnetic field.

8. Apparatus as defined inv claim 6 wherein said hollow metallic cylinder is a hollow cylinder of stainless steel and said gas is hydrogen. v

References Cited in the file of this patent UNITED STATES PATENTS Hines Aug. 26, Washburn et al. Jan. 27, 1953 Berry Mar. 17, 1953 Cuccia May-'12,. 1953 Haeif Sept. 17, 1957 

6. APPARATUS COMPRISING A HOLLOW METALLIC CYLINDER, AN ELECTRON BEAM COLLECTOR POSITIONED AT ONE END OF SAID CYLINDER, AN ELECTRON GUN POSITIONED AT THE OTHER END OF SAID CYLINDER FOR DIRECTING AN ELECTRON BEAM PARALLEL TO THE AXIS OF SAID CYLINDER TOWARD SAID BEAM COLLECTOR, MEANS CONNECTED TO SAID CYLINDER FOR ESTABLISHING A PRESSURE OF THE ORDER OF 10-4 MILLIMETERS OF MERCURY OF AN IONIZABLE GAS WITHIN SAID CYLINDER IN THE REGION BETWEEN SAID ELECTRON GUN AND SAID BEAM COLLECTOR WHEREBY A PLASMA IS FORMED IN SAID REGION UPON THE BOMBARDMENT OF SAID GAS BY SAID ELECTRON BEAM, MEANS FOR PRODUCING A MAGNETIC FIELD DIRECTED AXIALLY ALONG SAID CYLINDER, THE CHARGED PARTICLES COMPRISING SAID PLASMA OSCILLATING AT A FREQUENCY DETERMINED BY THE MAGNETIC OF SAID MAGNETIC 